Optoelectronic devices can convert radiant energy into electrical energy or vice versa. These devices generally include an active layer sandwiched between two electrodes, sometimes referred to as the front and back electrodes, at least one of which is typically transparent. The active layer typically includes one or more semiconductor materials. In a light-emitting device, e.g., a light-emitting diode (LED), a voltage applied between the two electrodes causes a current to flow through the active layer. The current causes the active layer to emit light. In a photovoltaic device, e.g., a solar cell, the active layer absorbs energy from light and converts this energy to electrical energy exhibited as a voltage and/or current between the two electrodes. Large scale arrays of such solar cells can potentially replace conventional electrical generating plants that rely on the burning of fossil fuels. However, in order for solar cells to provide a cost-effective alternative to conventional electric power generation the cost per watt generated may be competitive with current electric grid rates. Currently, there are a number of technical challenges to attaining this goal.
Most conventional solar cells rely on silicon-based semiconductors. In a typical silicon-based solar cell, a layer of n-type silicon (sometimes referred to as the emitter layer) is deposited on a layer of p-type silicon. Radiation absorbed proximate the junction between the p-type and n-type layers generates electrons and holes. The electrons are collected by an electrode in contact with the n-type layer and the holes are collected by an electrode in contact with the p-type layer. Since light may reach the junction, at least one of the electrodes may be at least partially transparent. Many current solar cell designs use a transparent conductive oxide (TCO) such as indium tin oxide (ITO) as a transparent electrode.
A further problem associated with existing solar fabrication techniques arises from the fact that individual optoelectronic devices produce only a relatively small voltage. Thus, it is often necessary to electrically connect several devices together in series in order to obtain higher voltages in order to take advantage of the efficiencies associated with high voltage, low current operation (e.g. power transmission through a circuit using relatively higher voltage, which reduces resistive losses that would otherwise occur during power transmission through a circuit using relatively higher current).
Several designs have been previously developed to interconnect solar cells into modules. For example, early photovoltaic module manufacturers attempted to use a “shingling” approach to interconnect solar cells, with the bottom of one cell placed on the top edge of the next, similar to the way shingles are laid on a roof. Unfortunately the solder and silicon wafer materials were not compatible. The differing rates of thermal expansion between silicon and solder and the rigidity of the wafers caused premature failure of the solder joints with temperature cycling.
A further problem associated with series interconnection of optoelectronic devices arises from the high electrical resistivity associated with the TCO used in the transparent electrode. The high resistivity restricts the size of the individual cells that are connected in series. To carry the current from one cell to the next the transparent electrode is often augmented with a conductive grid of busses and fingers formed on a TCO layer. However, the fingers and busses produce shadowing that reduces the overall efficiency of the cell. In order for the efficiency losses from resistance and shadowing to be small, the cells may be relatively small. Consequently, a large number of small cells may be connected together, which requires a large number of interconnects and more space between cells. Arrays of large numbers of small cells are relatively difficult and expensive to manufacture. Further, with flexible solar modules, shingling is also disadvantageous in that the interconnection of a large number of shingles is relatively complex, time-consuming and labor-intensive, and therefore costly during the module installation process.
To overcome this, optoelectronic devices have been developed with electrically isolated conductive contacts that pass through the cell from a transparent “front” electrode through the active layer and the “back” electrode to an electrically isolated electrode located beneath the back electrode. U.S. Pat. No. 3,903,427 describes an example of the use of such contacts in silicon-based solar cells. Although this technique does reduce resistive losses and can improve the overall efficiency of solar cell devices, the costs of silicon-based solar cells remains high due to the vacuum processing techniques used in fabricating the cells as well as the expense of thick, single-crystal silicon wafers. However, even these designs create some shadow loss and often involve more elaborate manufacturing techniques involving high accuracy alignment of the traces to be pringed.
Due to the aforementioned issues, improved techniques are desired so that improved photovoltaic cells are manufactured. Improvements may be made to increase the active area of the underlying solar cell without significantly reducing electrical conductivity. The improved device performance and increased production throughput should increase market penetration and commercial adoption of such products.